(1) Field of the Invention
The present invention relates to a process for suitably producing silicon nitride sintered bodies.
(2) Related Art Statement
According to a conventional process of producing silicon nitride sintered bodies, a sintering aid which forms a liquid phase at crystal boundaries, for instance, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO and SrO, is added to a powdery silicon nitride starting material and a resulting mixture is fired in N.sub.2 atmosphere at a low O.sub.2 partial pressure by using a carbonaceous heater and a carbonaceous firing jig. For instance, Japanese patent publication No. 58-49,509 discloses a method in which firing is carried out in a pressurized N.sub.2 atmosphere or a mixed pressurized atmosphere of N.sub.2 and an inert gas.
Furthermore, the sintered bodies are reheated to remove flaws on worked surfaces thereof. For instance, in order to improve the surface property, reheating is carried out in an N.sub.2 atmosphere or a pressurized N.sub.2 atmosphere in Japanese patent application Laid-open No. 60-151,290 while reheating is carried out in an oxidizing atmosphere in U.S. Pat. No. 4,457,958.
By the way, oxide additives and SiO.sub.2 which is inherently contained in a silicon nitride starting material and serve as a sintering aids form glass at an intergranular phase upon reaction, so that it effectively promotes the densification of a structure and forms microstructures. However, according to the above-mentioned conventional method, when a molding of silicon nitride is fired in an N.sub.2 atmosphere or an N.sub.2 pressurized atmosphere at a low partial pressure of O.sub.2, as shown in the following formulae (1) and (2), the oxide additive or SiO.sub.2 evaporates from the glass phase, or is nitrided. Consequently, contents of O and N in the glass phase vary to change the composition of the intergranular glass phase. EQU SiO.sub.2 evaporation reaction: SiO.sub.2 .revreaction.SiO+1/2O.sub.2( 1) EQU SiO.sub.2 nitriding reaction: 3SiO.sub.2 +2N.sub.2 .revreaction.Si.sub.3 N.sub.4 +3O.sub.2 ( 2)
For this reason, the conventional methods have the shortcomings that fully densified silicon nitride cannot be obtained; the microstructure becomes different between the surface at which evaporation is easy and the inside in which the evaporation is difficult; and the physical properties of the fired surface are deteriorated. In particular, in the case of silicon nitride sintered bodies added with one or more rare earth element oxides or silicon nitride sintered bodies added with Y.sub.2 O.sub.3 and/or MgO which have high temperature characteristics improved through crystallizing treatment of the intergranular glass phase, there was a shortcoming that intergranular crystals which have poor oxidation resistance are precipitated in the fired surface due to change in composition of the intergranular glass phase after the crystallization. The above-mentioned shortcomings are not improved by reheating the sintered body in the N.sub.2 atmosphere or the N.sub.2 pressurized atmosphere at a low O.sub.2 partial pressure. Further, reheating under completely oxidative atmosphere causes an oxidized layer to be formed on the surface, thereby deteriorating mechanical characteristics such as hardness, high temperature strength, etc. peculiar to the silicon nitride sintered bodies.
Moreover, when the sintered body is ground and worked after the removal of the surface layer having such poor characteristics or the sintered body is worked to expose the inside thereof (hereinafter referred to as "inside worked face"), for instance, so as to obtain a desired shape, minute cracks are formed on the inside worked face which lower the strength.
When the thus worked bodies are reheated in the N.sub.2 atmosphere or the N.sub.2 pressurized atmosphere having a low O.sub.2 partial pressure, a fired surface having a newly changed composition is formed to deteriorate the characteristics thereof.